1. Field of the Invention
The present invention relates to an image forming apparatus, such as a laser printer, a copying machine, or a facsimile, that employs electrophotographic technology. More particularly, the present invention relates to a member for retaining a gap between an image bearing member and a developer bearing member.
2. Description of the Related Art
Referring to FIGS. 2 and 3, an image forming apparatus 1 that employs electrophotographic technology selectively exposes a photosensitive drum 20, which is an image bearing member and has been uniformly charged by a charging device 30, by an exposure device 3 to thereby form a latent image on the photosensitive drum 20. The latent image is developed using a developer or toner T, and the developed image is transferred to a recording medium P, such as paper or OHT. Then, the transferred image on the recording medium P is pressed and thermally fixed using a fixing device 9 so as to record the image. Upon completion of transferring the image toner, the toner remaining on the photosensitive drum 20 is removed by a cleaning device 50 before another cycle of the image forming process beginning with electrical charging is started.
As the photosensitive drum 20 used with such an image forming apparatus, a photosensitive drum has been known, in which a conductive base material, such as aluminum, is employed as a base member 21, and a photoconductive material providing a photosensitive layer 22 is coated on the base member 21 as shown in FIG. 5. Conventionally used photoconductive materials include inorganic photoconductive materials, such as selenium, cadmium sulfide, and zinc oxide, and organic photoconductive materials, such as polyvinyl carbazole, oxadiazole, and phthalocyanine. When using an organic photoconductive material, in particular, it has been known to use, for the purpose of sensitization, the photosensitive layer 22 formed of a laminate of an electric charge generating layer 22a and an electric charge transporting layer 22b to separately provide different functions. The electric charge transporting 22b serving as a surface layer that uses polycarbonate as a binder has been put in practical use.
A developing device 40 that has been put in practical use is constituted by a toner chamber 45 holding a toner T, a developer supply chamber 44, a developer bearing member or a developing roller 41, and a developer restricting member or a developing blade 42 as shown in FIG. 3.
The developing roller 41 is formed of a conductive material, such as aluminum, and rotatably retained with a predetermined gap, preferably 0.2 mm to 0.5 mm, from the photosensitive drum 20. The developing roller 41 incorporates therein a magnet roller 43 having a plurality of magnetic poles. The magnet roller 43 is retained by a developing device frame 46 so that it does not rotate as shown in FIG. 5.
The developing blade 42 is secured to the developing device frame 46 such that one end thereof is pressed against an outer peripheral surface of the developing roller 41.
As shown in FIG. 3, the toner T of the toner chamber 45 is carried from the toner chamber 45 to the developer supply chamber 44 by its own weight or a carrying means (not shown), and drawn to the vicinity of the developing roller 41 by a magnetic force of the magnet roller 43. The toner T carried to the vicinity of the developing roller 41 is carried toward the developing blade 42 by the rotation of the developing roller 41, restricted in its layer thickness by the developing blade 42, then carried toward the photosensitive drum 20. At this point, a predetermined level of electric charge is imparted to the toner T having its layer thickness restricted when passing through a nipped portion between the developing blade 42 and the developing roller 41.
In the aforesaid developing device 40, means for stably holding the developing roller 41 and the photosensitive drum 20 with a predetermined gap therebetween is formed of polyacetal, and has substantially cylindrical gap retaining members or spacer rollers 60 having a wall thickness of, for example, 0.2 mm to 0.5 mm. The spacer rollers 60 are rotatably fitted at both ends of the developing roller 41, and the developing roller 41 is pressed toward the photosensitive drum 20 by a compression spring 54 functioning as an urging means, as shown in FIGS. 1 and 4. Two typical examples are shown in FIG. 1.
Extended use of the image forming apparatus 1 requires supply of the toner T and adjustment, cleaning, or replacement of the photosensitive drum 20 and other means for implementing diverse processes. Such maintenance and service have been difficult in practice for people other than servicemen with expertise.
To overcome the difficulty mentioned above, a process cartridge 2 shown in FIG. 2, for example, has been devised and put in practical use. The process cartridge 2 combines processing means, such as the toner T, the photosensitive drum 20, the developing device 40, the charging device 30, and the cleaning device 50, into one unit. The process cartridge 2 is detachably installed on the image forming apparatus 1, thus improving maintainability.
With recent dissemination of the image forming apparatus 1, there have been demands for a reduced size, a reduced noise, and lower cost of the image forming apparatus (apparatus main body) 1. The reduction of noise and cost has been studied and achieved by obviating the need for an exhaust fan. As other measures for achieving the lower cost, the material used for a main body frame 1a has been changed from a sheet metal to a resin to permit one-piece molding. However, an attempt to achieve the reduced size, the controlled noise, and the lower cost of the apparatus main body 1 employing the conventional spacer rollers 60 has been facing difficulties set forth below.
The apparatus main body 1 includes various heat generating sources, including the fixing device 9. Making the apparatus main body 1 smaller would cause the temperature in the apparatus main body 1 to rise more easily. Especially in the case of the apparatus main body 1 not equipped with an exhaust fan, the temperature in the apparatus main body 1 would rise more easily.
The spacer rollers 60 are repeatedly subjected to compressive stress. Hence, if the apparatus main body 1 is made smaller or not provided with an exhaust fan, then the spacer rollers 60 will be repeatedly subjected to compressive stress at a high temperature. In the case of an apparatus that employs the spacer rollers 60 that can be fitted at the ends of the developing roller 41 as shown in FIG. 1, there has been a danger in that the spacer rollers 60 incur rolling deformation. As a result, prolonged use of the apparatus may cause the spacer rollers 60 to fail to maintain a proper gap.
In addition, the outer peripheries of the spacer rollers 60 are usually pressed against the photosensitive layer 22 of the photosensitive drum 20. The spacer rollers 60 are repeatedly brought into contact with and moved away from the photosensitive layer 22, and the friction from sliding against each other causes the outer peripheral surface of the spacer rollers 60 to be easily charged.
The photosensitive layer 22 of the photosensitive drum 20 is preferably short from the viewpoint of manufacturing process and cost. For this reason, it is desirable to expose the base member 21 of the photosensitive drum 20 at both end areas not involved in formation of images as shown in FIG. 5 rather than providing those end areas also with the photosensitive layer 22.
Because of the exposed areas, there have been cases where the surfaces of the spacer rollers 60 are charged, and the electric charges are discharged to exposed portions 21a of the base member of the photosensitive drum 20, radiating a field emission voltage. There has been a danger in that the field emission voltage passes through a resinous frame and radiates out of an apparatus main body in some cases. Therefore, depending on a combination of the spacer rollers 60 and the photosensitive layer 22, it has been required to provide the apparatus main body 1 with an electrical shielding by, for example, providing a sheet metal component for shielding in order to prevent field emission voltages from being radiated out of the apparatus main body 1. This has been interfering with achieving a reduced cost of the apparatus main body 1.